I should perhaps note that we have been changing the published date as a hacky sort of way to change the post display order. I'm not sure when the event started being omitted, or if this is related, but there it is.

Does anyone have any insight as to why this kind of thing would happen? Thanks.

I wouldn't recommend using the publishing date as the date for the event's occurrence - it would be better (and easier) to save this as post meta or stored in a custom table.
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Stephen HarrisFeb 23 '12 at 23:44